fix(decode): accept signed enum tag scalars in _extract_tag (#954)

## Summary

- Relax `_extract_tag` to accept both signed and unsigned `PrimitiveInt`
tag metadata (`signed=_`).
- Keep discriminant decoding based on raw tag bits (`int.from_bytes(...,
signed=False)`), which is what variant dispatch needs.
- Add a decode fixture for signed-tag metadata
(`enum-3-variants-0-fields-signed-tag`).
- Update the prove-rs case to a passing case and refresh its `show`
snapshot.

## Problem (concrete case)

This PR is motivated by:


`kmir/src/tests/integration/data/prove-rs/transmute-u8-to-enum-changed-discriminant-signed.rs`

```rust
#[repr(i8)]
enum AccountState {
    Uninitialized = 0,
    Initialized = 1,
    Frozen = -1,
}

// main checks both byte-level and signed views of the same discriminant bits:
// 255 maps to Frozen in the u8 path
// -1 maps to Frozen in the i8 path
```

For this enum layout, the tag scalar metadata can be `signed=true` (I8),
even though the stored discriminant bytes are still valid raw bytes for
dispatch.

## Red (before this fix)

`_extract_tag` only matched `PrimitiveInt(..., signed=False)`. When
metadata was signed, decode failed with:

```text
ValueError: Unsupported tag: Initialized(value=PrimitiveInt(length=I8, signed=True), ...)
```

That blocked the signed-discriminant path in proving.

## Green (after this fix)

- `_extract_tag` now accepts either signedness metadata.
- The same bytes are decoded as unsigned raw bits for dispatch.
- The concrete program above now proves in integration tests:
- input case is now
`transmute-u8-to-enum-changed-discriminant-signed.rs`
- expected proof snapshot is
`show/transmute-u8-to-enum-changed-discriminant-signed.main.expected`
  - terminal `#EndProgram` is reached for `main`

## Why this is the right scope

This change fixes tag extraction compatibility for signed discriminant
metadata without changing enum dispatch semantics beyond that path. It
is covered by both:

- decode-value fixture (`enum-3-variants-0-fields-signed-tag`) and
- end-to-end prove-rs integration snapshot for the concrete Rust program
above.

---------

Co-authored-by: automergerpr-permission-manager[bot] <190534181+automergerpr-permission-manager[bot]@users.noreply.github.com>

Author: Stevengre

kmir/src/kmir/decoding.py

@@ -445,10 +445,12 @@ def _extract_tag(*, data: bytes, tag_offset: MachineSize, tag: Scalar) -> tuple[
         case Initialized(
             value=PrimitiveInt(
                 length=length,
-                signed=False,
+                signed=_,
             ),
             valid_range=_,
         ):
+            # Stable MIR enum discriminants are represented against the raw tag bits.
+            # Use unsigned decoding even when the scalar metadata marks the tag as signed.
             tag_data = data[tag_offset.in_bytes : tag_offset.in_bytes + length.value]
             tag_value = int.from_bytes(tag_data, byteorder='little', signed=False)
             return tag_value, length

kmir/src/tests/integration/data/decode-value/enum-3-variants-0-fields-signed-tag.expected

@@ -0,0 +1 @@
+Aggregate ( variantIdx ( 1 ) , .List )

kmir/src/tests/integration/data/decode-value/enum-3-variants-0-fields-signed-tag.json

@@ -0,0 +1,138 @@
+{
+  "bytes": [
+    0
+  ],
+  "types": [],
+  "typeInfo": {
+    "EnumType": {
+      "name": "OrderingLike",
+      "adt_def": 72,
+      "discriminants": [
+        255,
+        0,
+        1
+      ],
+      "fields": [
+        [],
+        [],
+        []
+      ],
+      "layout": {
+        "fields": {
+          "Arbitrary": {
+            "offsets": [
+              {
+                "num_bits": 0
+              }
+            ]
+          }
+        },
+        "variants": {
+          "Multiple": {
+            "tag": {
+              "Initialized": {
+                "value": {
+                  "Int": {
+                    "length": "I8",
+                    "signed": true
+                  }
+                },
+                "valid_range": {
+                  "start": 255,
+                  "end": 1
+                }
+              }
+            },
+            "tag_encoding": "Direct",
+            "tag_field": 0,
+            "variants": [
+              {
+                "fields": {
+                  "Arbitrary": {
+                    "offsets": []
+                  }
+                },
+                "variants": {
+                  "Single": {
+                    "index": 0
+                  }
+                },
+                "abi": {
+                  "Aggregate": {
+                    "sized": true
+                  }
+                },
+                "abi_align": 1,
+                "size": {
+                  "num_bits": 8
+                }
+              },
+              {
+                "fields": {
+                  "Arbitrary": {
+                    "offsets": []
+                  }
+                },
+                "variants": {
+                  "Single": {
+                    "index": 1
+                  }
+                },
+                "abi": {
+                  "Aggregate": {
+                    "sized": true
+                  }
+                },
+                "abi_align": 1,
+                "size": {
+                  "num_bits": 8
+                }
+              },
+              {
+                "fields": {
+                  "Arbitrary": {
+                    "offsets": []
+                  }
+                },
+                "variants": {
+                  "Single": {
+                    "index": 2
+                  }
+                },
+                "abi": {
+                  "Aggregate": {
+                    "sized": true
+                  }
+                },
+                "abi_align": 1,
+                "size": {
+                  "num_bits": 8
+                }
+              }
+            ]
+          }
+        },
+        "abi": {
+          "Scalar": {
+            "Initialized": {
+              "value": {
+                "Int": {
+                  "length": "I8",
+                  "signed": true
+                }
+              },
+              "valid_range": {
+                "start": 255,
+                "end": 1
+              }
+            }
+          }
+        },
+        "abi_align": 1,
+        "size": {
+          "num_bits": 8
+        }
+      }
+    }
+  }
+}

kmir/src/tests/integration/data/prove-rs/show/transmute-u8-to-enum-changed-discriminant-signed-fail.main.expected

@@ -54,7 +54,6 @@
     'assume-cheatcode-conflict-fail',
     'raw-ptr-cast-fail',
     'transmute-u8-to-enum-fail',
-    'transmute-u8-to-enum-changed-discriminant-signed-fail',
     'assert-inhabited-fail',
     'iterator-simple',
     'unions-fail',